1. Field of the Invention
The present invention relates generally to a simple matrix liquid crystal display for use in information equipment, AV (Audio Visual) equipment, displays for advertising purposes or the like, and more particularly, to a liquid crystal display having an adjustable effective voltage value applied to the liquid crystal panel for display.
2. Description of the Related Art
A simple matrix liquid crystal display has a liquid crystal display panel having a plurality of pixels arranged in a matrix. The liquid crystal display panel has a pair of transparent substrates opposite to each other with a layer of liquid crystal material therebetween. In one of the transparent substrates, a plurality of scanning electrodes are provided parallel to each other, and in the other transparent substrate, a plurality of signal electrodes are provided parallel to each other and orthogonal to the scanning electrodes. A pixel is sequentially selected by applying a selecting pulse from a scanning circuit to each scanning electrode, and a signal potential corresponding to the display data of the selected pixel is applied to a corresponding signal electrode for display.
In the simple matrix liquid crystal display, irregularity in the thickness of cells in the display panel, or the characteristic of orientation of the liquid crystal material could cause difference in the optimum voltage value between the central portion and the end portion of the liquid crystal display panel. Heat generated by the back light lamp causes change in the characteristics of liquid crystal material, and therefore the optimum voltage value is different between various parts of the liquid crystal panel depending upon the distance from the lamp. Such difference in the optimum voltage value on the liquid crystal display panel causes unevenness in display quality because of voiding phenomenon corresponding to the difference. This will be described in further detail. FIG. 20 is a graph showing electro-optical characteristics of a conventional liquid crystal display panel at the central and the end portion. Referring to FIG. 20, if the effective voltage value for obtaining an optimum black display is Vthm at the central part of the liquid crystal display panel and Vthl at the end portion, a voiding phenomenon is generated at the end portion in response to application of voltage at the level of effect voltage value (Vthm), because the transmissivity is larger at the end portion of the liquid crystal panel than at the central portion.
In order to solve the above-described problem, Japanese Patent Laying-Open No. 7-20483 discloses a method of improving unevenness in display quality caused by difference in the electro-optical characteristic of a liquid crystal display panel for a single matrix liquid crystal display between at its end portion and central portion because of irregularity in cell thickness and the orientation of liquid crystal material generated in the manufacture. Normally, when a liquid crystal display element is charged/discharged, the waveform of voltage actually applied to the liquid crystal material has a distortion depending upon the value of resistance of the lead wire portion of each scanning electrode and each signal electrode or the value of the internal resistance of a driving circuit, which changes the value of effective voltage applied to the liquid crystal material. In the above-described method, the value of resistance of the lead wire portion is made different between each scanning electrode and each signal electrode to change the degree of distortion of the waveform of voltage generated at each electrode, and effective voltage at an appropriate level is supplied to each part of the liquid crystal layer.
Japanese Patent Laying-Open No. 8-201779 discloses a method of improving unevenness in display quality caused by difference in the electro-optical characteristic of a liquid crystal display panel between the end portion and the central portion, because of the influence of heat generated from a lamp used for an edge light type back light. According to the method, the potential of a selecting pulse to be supplied to each scanning electrode is controlled and changed depending upon the distribution of temperature on the liquid crystal display panel which is predicted for each scanning electrode, and effective voltage at a more appropriate level is supplied to each part of the liquid crystal layer.
According to the method disclosed by Japanese Patent Laying-Open No. 7-20483, change in the effective voltage value caused by a distortion in the waveform of voltage applied to the liquid crystal layer which is generated at the time of charging/discharging the liquid crystal layer is adjusted by changing the value of resistance at each electrode. Therefore, change in the effective voltage value is disadvantageously affected by the number of inversion of the polarity of potential applied to a signal electrode. More specifically, the amount to vary the value of resistance at each electrode changes depending upon the display pattern at the liquid crystal display panel, and such method depending upon the display pattern is not preferable, if the conventional driving method is applied.
According to the method disclosed by Japanese Patent Laying-Open No. 8-201779, principally no difference appears in the effect between display patterns, because a distortion in the waveform of potential is not utilized. However, the output of a voltage supply circuit to the liquid crystal display panel should be continuously changed analog-wise from time to time, the potential supply cannot be stabilized with a simple circuit configuration, and therefore the method is subject to the influence of a distortion in the waveform of potential generated at the time of charging/discharging the liquid crystal layer, which can cause increased crosstalk.
It is an object of the invention to provide a liquid crystal display capable of applying effective voltage at an optimum level to each part of a liquid crystal display panel without using the effect of change in the value of effective voltage caused by charging/discharging the liquid crystal display panel or without changing analog-wise potential applied to each electrode in the liquid crystal display panel from time to time.
In order to achieve the above-described object, a liquid crystal display according to the present invention includes a liquid crystal display panel including a signal side substrate having an arrangement of a plurality of signal electrodes, a scanning side substrate disposed opposite to the signal side substrate and having an arrangement of a plurality of scanning electrodes crossing the plurality of signal electrodes and a liquid crystal layer between the signal side substrate and scanning-side substrate, and a signal side driving circuit and a scanning side driving circuit for applying driving pulse voltage signals to display data to the plurality of signal electrodes and the plurality of scanning electrodes, respectively for driving. In the display, at least one of the driving pulse voltage signals applied to the plurality of signal electrodes and the plurality of scanning electrodes is corrected by applying a correction pulse voltage signal such that the value of effective voltage at each part of the liquid crystal display panel attains an optimum level.
Therefore, the value of effective voltage applied to each part on the liquid crystal display panel may be corrected into an optimum level by applying the correction pulse voltage signal to the driving pulse voltage signal applied to each part. Hence, unevenness in display quality caused by the inability of supplying effective voltage at an appropriate level at each part of the liquid crystal display panel may be improved.
The pulse width of the correction pulse voltage signal can be adjusted such that the value of effective voltage applied to each part of the liquid crystal panel is at an optimum level for at least one of the plurality of scanning electrodes and the plurality of signal electrodes.
The pulse width of the correction pulse voltage signal may be adjusted for at least one of each scanning electrode and each signal electrode to set the effective voltage value of each part of the liquid crystal display panel at an optimum level. As a result, unevenness in display quality caused by the inability of obtaining optimum effective voltage at each part on the liquid crystal display panel may be improved without dependence on the display pattern or without increase in crosstalk.
The driving pulse voltage signal described above includes a data voltage signal applied to the plurality of signal electrodes for displaying data, a selecting voltage signal applied to the plurality of scanning electrodes in the selecting periods for scanning while selecting the plurality of scanning electrodes when data is displayed, and the correction pulse voltage signal includes a non-selecting voltage signal at a different level from the selecting voltage signal and applied in the non-selecting periods of the plurality of scanning electrodes. The non-selecting voltage signal is applied for a first prescribed time period to the plurality of scanning electrodes while the scanning electrodes are selected, and the first prescribed time period can be adjusted for the plurality of scanning electrodes.
The pulse width of the selecting voltage signal may be adjusted for each scanning electrode such that the effective voltage value of each part of the liquid crystal display panel attains an optimum level. Therefore, unevenness in display quality caused by the inability of obtaining an optimum effective voltage value at each part of the liquid crystal display may be improved without dependence on the display pattern or without crosstalk.
The correction pulse voltage signal includes an intermediate voltage signal applied to the plurality of signal electrodes when data is not displayed. The intermediate voltage signal is applied to the plurality of signal electrodes if data voltage signal is applied for a second prescribed time period, and the second prescribed time period can be adjusted for the plurality of signal electrodes.
Therefore, the second prescribed time period during which the intermediate voltage signal is applied to the plurality of signal electrodes may be adjusted for each signal electrode such that an optimum level effective voltage value may be obtained at each part of the liquid crystal display panel. As a result, unevenness in display quality caused by the inability of obtaining the optimum level effective voltage value at each part of the liquid crystal display panel may be improved without dependence on the display pattern or without increase in crosstalk.
The pulse width of the correction pulse voltage signal may be further adjusted following temperature changes in the liquid crystal display panel for at least one of the plurality of scanning electrodes and the plurality of signal electrodes.
As a result, the level of effective voltage applied to each part on the liquid crystal display panel may be adjusted into an optimum level based on temperature changes in the liquid crystal display panel. Consequently, unevenness in display quality caused by temperature changes in the liquid crystal display panel may be improved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.